A Kiwi shooting friend recently asked me if I'd write a blog post on my load development process. I replied by Facebook message that I would be delighted to; although I did warn him that it might not necessarily be quite what he was expecting. Here's to you CG.
I find out the starting load for the powder I want to use, load 5 rounds each for loads starting from the starting load to the maximum load listed in the reloading manuals in 0.5 grain increments. I shoot them in sequence, checking for pressure signs* to determine the actual max load in my rifle.
Back off about 1 grain to allow a general safety margin and some headroom for temperature variation. That's your long range load for 900x and 1000x. Back off 1 more grain. That's your short range load for 300x to 600x.
If possible check the loads at any short range and 1000x before you use them in anger, and chrono them if you can, but in my experience it's rare that you will get a load that just won't perform acceptably at the very least if you're shooting a 30" barrel.
1) 308 Winchester / 7.62 x 51** is a pretty easy calibre to load for, and when using a good quality bullet, case, primer and powder your loads aren't going to differ that much in accuracy in the normal range of loads listed in your average reloading manual. That variation in accuracy is probably going to be much smaller than you are able to detect, given that the best group that a "top 25 in the world" international shot will be able to hold consistently is about 1 moa at 1000 yards***.
2) Provided that you're getting over 2925 fps then you're probably going to be supersonic at 1000 yards with most sensible bullets, and the additional velocity you're going to get from using a double-base powder isn't going to make that much of a difference to the wind-bucking characteristics of the round. Using double-base powders will give you a velocity boost, but at the cost of a seriously reduced barrel life.
3) It probably costs you something like GBP1.50 - 3.00 or NZD2.50 - 5.00 every time you pull the trigger when you account for barrel life, ammunition cost, target hire, fuel and accommodation. Would you prefer to spend that money finding the perfect load, or getting some focussed range time in?
In summary, the time, cost and effort of working out the perfect load for your rifle just isn't worth it for the vast majority of TR shooters because the difference between the accuracy of the rifle/ammunition combination and the accuracy of the shooter is too great (although it is probably worth it for our F-TR and F-Open shooting cousins.)
Example Short/Long Range Loads
VihtaVuori N140 - 45.0/46.0 grains
VihtaVuori N150 - 47.0/48.0 grains
TR 140 - 45.5/46.5 grains
Varget / ADI AR2208 - 45.0/46 grains
Case: Neck sized RWS brass
Primer: Federal 210 Match or Remington 9 1/2
Bullet: 30 Calibre Sierra MatchKing #2156 (40 thou jump)
NB - These loads have been proved to be safe in my rifle, but they may not be your rifle so please work up your loads responsibly. If you fail to do so, you might get lucky and not have a problem, but then again you may end up amputating various portions of your left hand when your barrel lets go and/or potentially blinding yourself with the gas and brass fragment blowback. If you're lucky. That's not to mention the damage you could do to your fellow shooters and innocent bystanders. It could also trigger nuclear conflict between previously friendly nations. So there. Don't be daft.
* Pressure signs include, but are not limited to: Excessive muzzle blast, cratered primers, flattened primers, sooty marks from as leaks around primers, loose primers, blown primers, and possibly most importantly case head expansion. This list is not exhaustive.
** Yes. I know that these aren't quite the same thing, strictly speaking.
** I know this to be a fact because I spent quite a few hours in the summer of 2008 measuring, recording into Excel and then analysing the details of all ~1500 shots fired by the winning GB Palma Team in the 2007 Palma Match. The shooter with the smallest statistical group held 0.96 minutes of arc with a confidence interval of 95%, the shooter with the biggest was about 1.60 minutes.